[linux] fix theme updates on gnome 42

Read `org.gnome.desktop.interface.color-scheme` if available. The old
brightness detection algorithm is kept as a fallback for older gnome and
other platforms.

Fixes: flutter/flutter#101438

Author: jpnurmi

shell/platform/linux/fl_settings_plugin.cc

@@ -17,10 +17,13 @@ static constexpr char kAlwaysUse24HourFormatKey[] = "alwaysUse24HourFormat";
 static constexpr char kPlatformBrightnessKey[] = "platformBrightness";
 static constexpr char kPlatformBrightnessLight[] = "light";
 static constexpr char kPlatformBrightnessDark[] = "dark";
+static constexpr char kPreferLight[] = "prefer-light";
+static constexpr char kPreferDark[] = "prefer-dark";
 
 static constexpr char kDesktopInterfaceSchema[] = "org.gnome.desktop.interface";
 static constexpr char kDesktopTextScalingFactorKey[] = "text-scaling-factor";
 static constexpr char kDesktopClockFormatKey[] = "clock-format";
+static constexpr char kDesktopColorSchemeKey[] = "color-scheme";
 static constexpr char kClockFormat24Hour[] = "24h";
 
 enum class Brightness { Light, Dark };
@@ -89,18 +92,31 @@ static bool is_dark_theme() {
 static void update_settings(FlSettingsPlugin* self) {
   gdouble scaling_factor = 1.0;
   gboolean always_use_24hr = FALSE;
-  const gchar* platform_brightness = kPlatformBrightnessLight;
+  const gchar* platform_brightness = nullptr;
 
   if (self->interface_settings != nullptr) {
     scaling_factor = g_settings_get_double(self->interface_settings,
                                            kDesktopTextScalingFactorKey);
     g_autofree gchar* clock_format =
         g_settings_get_string(self->interface_settings, kDesktopClockFormatKey);
     always_use_24hr = g_strcmp0(clock_format, kClockFormat24Hour) == 0;
+
+    GSettingsSchema* schema = nullptr;
+    g_object_get(self->interface_settings, "settings-schema", &schema, nullptr);
+    if (g_settings_schema_has_key(schema, kDesktopColorSchemeKey)) {
+      g_autofree gchar* color_scheme = g_settings_get_string(
+          self->interface_settings, kDesktopColorSchemeKey);
+      if (g_strcmp0(color_scheme, kPreferLight) == 0) {
+        platform_brightness = kPlatformBrightnessLight;
+      } else if (g_strcmp0(color_scheme, kPreferDark) == 0) {
+        platform_brightness = kPlatformBrightnessDark;
+      }
+    }
   }
 
-  if (is_dark_theme()) {
-    platform_brightness = kPlatformBrightnessDark;
+  if (platform_brightness == nullptr) {
+    platform_brightness =
+        is_dark_theme() ? kPlatformBrightnessDark : kPlatformBrightnessLight;
   }
 
   g_autoptr(FlValue) message = fl_value_new_map();
@@ -155,7 +171,7 @@ void fl_settings_plugin_start(FlSettingsPlugin* self) {
   GSettingsSchemaSource* source = g_settings_schema_source_get_default();
   if (source != nullptr) {
     g_autoptr(GSettingsSchema) schema =
-        g_settings_schema_source_lookup(source, kDesktopInterfaceSchema, FALSE);
+        g_settings_schema_source_lookup(source, kDesktopInterfaceSchema, TRUE);
     if (schema != nullptr) {
       self->interface_settings = g_settings_new_full(schema, nullptr, nullptr);
       gulong new_connections[] = {
@@ -168,6 +184,9 @@ void fl_settings_plugin_start(FlSettingsPlugin* self) {
           g_signal_connect_object(
               self->interface_settings, "changed::gtk-theme",
               G_CALLBACK(update_settings), self, G_CONNECT_SWAPPED),
+          g_signal_connect_object(
+              self->interface_settings, "changed::color-scheme",
+              G_CALLBACK(update_settings), self, G_CONNECT_SWAPPED),
       };
       g_array_append_vals(self->connections, new_connections,
                           sizeof(new_connections) / sizeof(gulong));